The present invention relates generally to photopolymerizable printing plates useful, for example, in letterpress and related printing operations, and more particularly to shallow relief, non-bottoming photopolymer printing plates and methods for making and using such plates.
Photopolymer printing plates have found widespread and successful use in letterpress printing processes, particularly in the newspaper industry. Conventional photopolymer plates hold many advantages over prior art, metal etched printing plates. The time required to make the photopolymer plates, for example, is considerably shorter and, with the introduction of water developable photopolymers, problems relating to environmental contamination have been significantly reduced. In addition, photopolymer plates are much easier to handle and can be more readily and efficiently developed and processed than metal etched plates.
Despite their widespread acceptance in the industry, however, photopolymer plates do suffer from the disadvantage of being relatively expensive, particularly when compared to the plates used in stereotype systems utilized by some of the large newspapers. Thus, there is a need in the industry for a less expensive photopolymer printing plate which will enable photopolymer printing systems to more efficiently compete with existing stereotype systems.
Conventional photopolymer printing plates utilize photosensitive materials which are deposited on a supporting substrate such as metal or plastic. Experience has shown that acceptable printing quality can often only be accomplished when such photopolymer plates utilize photosensitive layers having a thickness greater than 0.020 inch. Without such relatively thick photopolymer layers, and the resultant high relief image areas that they produce, "bottoming", e.g., the unwanted printing on white or non-image background areas, often results when thinner plates are used in letterpress machines.
Although a number of techniques have evolved in an effort to solve this "bottoming" problem, none has proven to be entirely satisfactory. Obviously, the use of thick photopolymer layers of 0.020 inch or more is undesirable because of the added expense that is caused through use of more photopolymer.
In some cases, ink repellant materials or even separate layers of ink repellant compositions have been incorporated in thinner photopolymer plates so that the non-image, background areas after development tend to reject any unwanted accumulation and subsequent deposit of ink in white or background areas. The disadvantage of such techniques, of course, is that significant additional expense is added to the resultant plate (even where thinner photopolymer layers can be employed) because of the special ink repulsive layers, additional materials and additional manufacturing costs that are required.
In other cases, highly expensive special printing presses and extreme care in printing are required to minimize the bottoming problem. Attempts have even been made to overcome the bottoming problem by depositing a thin layer of photopolymer, exposing the first layer of photopolymer with a screen dot negative to create a series of small polymerized areas for background, and then depositing a second layer of photopolymer material over the first layer for use in creating image areas. Such techniques, although partially useful in reducing the overall thickness of the resultant photopolymer layer, have the disadvantage of adding significant expense and of unnecessarily complicating the plate manufacturing process.
Finally, grained substrates have heretofore been used in the printing arts for purposes other than the prevention of "bottoming" in photopolymer plates, but these teachings are of little or no value in the context of the present invention. Grained substrates have been used in lithographic plates, for example, to aid in making improved water receptive surfaces. Similarly, grained substrates have been used to strengthen metal printing foils or the like so that localized deformations caused by means of a typewriter, pen, pencil, embossing plate or the like will not cause the foil to be split, torn or creased.
The problem, of course, with all such prior art techniques is that: (1) they fail even to recognize the nature and extent of "bottoming" problems that can occur in relatively thin photopolymer plates, and (2) they fail to provide a practical, inexpensive solution to the "bottoming problem, and more specifically a solution that permits careful, but simplified, control over the height, size, density and spacing of background protuberances which applicants have found useful in eliminating the "bottoming" problem.
In accordance with the present invention, therefore, a photopolymer printing plate is provided having a photopolymer layer that is substantially less thick, and thus far less expensive, than prior art photopolymer printing plates. Moreover, the printing plates of the present invention can not only be easily manufactured without adding significant time and expense to normal manufacturing techniques, but can be used on letterpress machines to produce printing material of high quality without unwanted bottoming occuring in background areas.
The present invention, therefore, is generally directed to shallow relief, non-bottoming photopolymer printing plates comprising (a) a substrate, (b) a binder layer coated on the substrate having a plurality of selected dispersed particles that create an array of selected protuberances in the background areas of the plate, and (c) a relatively thin photopolymer layer that is coated on the binder layer, which upon development, provides the desired raised image or relief areas of the resultant plate. The present invention is further directed to photopolymerizable elements, to methods for making and processing such elements to provide the desired shallow relief, non-bottoming printing plates, and to printing processes which advantageously utilize the shallow relief, non-bottoming plates of the present invention.